This invention relates generally to thermoplastic, packaging films, and more particularly to a shrinkable film with improved heat-shrinkability properties, optics or strength. In certain multilayer embodiments, the films also have improved sealing characteristics, such as easier sealability and improved seal strength, particularly in the presence of grease.
Shrinkable thermoplastic films have many useful applications in the packaging industry for food and non-food products. A heat shrinkable bag can be made from such films that has widespread use for meat and dairy products. Multilayer heat shrinkable films commonly contain ethylene-vinyl-acetate copolymers in one or more layers due to its toughness and low temperature shrinking characteristics.
A packaging film known by U.S. Pat. No. 3,741,253 comprises a core layer of a vinylidene chloride copolymer (commonly known as saran) between a layer of ethylene-vinyl acetate copolymer and a layer of cross-linked ethylene-vinyl acetate copolymer. Vinylidene chloride copolymer is abbreviated as PVDC and ethylene vinyl acetate copolymer as EVA.
A method for producing a film of EVA and PVDC is disclosed in U.S. Pat. No. 3,953,557 wherein the EVA has a narrow molecular weight distribution. The saran composition is a blend of emulsion and suspension polymerized saran or is a saran composition that includes emulsion polymerized vinylidene chloride of the type generally considered suitable for liquid coating.
U.S. Pat. No. 4,352,844, commonly assigned herewith to W. R. Grace & Co.-Conn., discloses a multilayer, heat shrinkable, thermoplastic film which comprises at least two EVA layers, the first EVA layer being a surface layer and the second layer being adjacent thereto. The EVA copolymer of the first layer has a higher melting temperature than the EVA copolymer of the second layer. The EVA copolymers in both layers are cross-linked and each layer is stretch oriented. The first and second layers form the substrate to which other layers can be added.
In one embodiment, the film has third and fourth layers wherein the third layer is a barrier layer that comprises a thermoplastic material having low gas permeability such as vinylidene chloride copolymer or hydrolyzed ethylene-vinyl acetate copolymer (hereinafter abbreviated as EVOH, and also known as ethylene-vinyl alcohol copolymer) and the fourth layer comprises an ethylene-vinyl acetate copolymer.
U.S. Pat. No. 4,640,856, which is commonly assigned herewith to W. R. Grace & Co.-Conn. and incorporated herein by reference, discloses multilayer films having a heat sealing VLDPE (very low density polyethylene) layer, an oxygen barrier layer, and at least a further outer layer, preferably a VLDPE layer. The patent discloses a multilayer thermoplastic barrier film having at least three layers comprising: (a) a layer consisting essentially of very low density polyethylene having a density of less than 0.910 gms/cc; (b) a barrier layer comprising a material selected from the group consisting of: (1) copolymers of vinylidene chloride and (2) hydrolyzed ethylene-vinyl acetate copolymers; (c) a thermoplastic polymeric layer, said layer being on the side of the barrier layer opposite to that of layer (a); and (d) the shrinkage of layer (a) controlling the shrinkage of the entire multilayer barrier film, said multilayer film having been oriented and rendered heat shrinkable at a temperature below 100.degree. C. (212.degree. F.), said orientation temperature being about 40.degree. F. or more below the melt temperature of the very low density polyethylene.
U.S. Pat. No. 4,801,486 also to W. R. Grace & Co.-Conn. discloses a thermoplastic multilayer packaging film comprising at least five layers in which one surface layer is a heat sealing layer and one of the internal layers is a barrier layer. The heat sealing surface layer comprises a copolymer of ethylene and an alpha-olefin having four to eight carbon atoms per molecule and the copolymer comprises ninety percent to seventy-five percent ethylene and ten percent to twenty-five percent alpha-olefin having four to eight carbon molecules and a density of less than 915 kg/m.sup.2. This material is called very low density linear polyethylene (VLDPE). Additionally, the heat sealing layer may comprise a blend of the copolymer above with an ethylene/vinyl acetate copolymer or an ethylene/butyl-acrylate copolymer.
A packaging film suitable for making bags and pouches and commonly assigned to W. R. Grace & Co.-Conn. is disclosed in U.S. Pat. 4,837,084, wherein a multilayer heat shrinkable film has at least one layer comprising a copolymer of ethylene and an alpha-olefin with six or more carbon atoms per molecule with the VLDPE copolymer having a density of about 0.910 g/cc or less and a melt index of about 2 or less. The entire disclosure of U.S. Pat. No. 4,837,084 is incorporated herein by reference.
A variety of embodiments of heat-shrinkable films are disclosed in U.S. Pat. No. 4,863,769 wherein in one embodiment the film may be a biaxially oriented monolayer film of a very low density polyethylene copolymer (VLDPE) with a density under 0.910 g/cc. In another embodiment, the film may be a biaxially oriented multilayer film comprising a first outer layer of an ethylene-vinyl acetate copolymer, a core layer of a barrier material such as a polyvinylidene chloride copolymer or an ethylene-vinyl alcohol copolymer, and a second outer layer comprising a blend of an ethylene-vinyl acetate copolymer and a very low density polyethylene copolymer with a density under 0.910 g/cc. The films are made by the double bubble method of Pahlke, U.S. Pat. No. 3,555,604.
U.S. Pat. No. 4,976,898, which is a Divisional of U.S. Pat. No. 4,863,769, discloses a method of manufacturing a heat shrinkable biaxially oriented thermoplastic film by extruding a primary tube, including very low density polyethylene with a density under 0.910 g/cc, heating the primary tube and biaxially stretching the heated primary tube using the double bubble method of Pahlke, U.S. Pat. No. 3,555,604, to form a tube which is heat-shrinkable.
Another multilayer film structure is disclosed in U.S. Pat. No. 5,041,316, assigned to W. R. Grace & Co.-Conn., wherein there is taught a multilayer thermoplastic shrinkable film with improved processing properties comprising at least a heat sealing VLDPE layer, a core oxygen barrier layer, an outer thermoplastic layer and also comprising a further intermediate VLDPE layer between the barrier and the outer thermoplastic layer.
A biaxially oriented, heat shrinkable film is disclosed by U.S. Pat. No. 5,059,481, which is a Continuation of U.S. Pat. No. 4,976,898, the film comprising VLDPE copolymer which comprises ethylene copolymerized with higher alpha olefins containing from four to eight carbon atoms such as butene, pentene, hexene, heptene and octene, and which has a density under 0.910 g/cc.
European Patent Application 91104044.2, (Publication No. 0447988, published Sep. 25, 1991), Forloni, Fornasiero, and Parnell, assignors to W. R. Grace & Co.-Conn., discloses a multilayer thermoplastic film from which containers may be made, said film having a thermosealing layer, a gas barrier layer, and at least one additional polymeric layer characterized in that the thermosealing layer can be sealed to itself to form a container with an opening through which a product may be loaded into the container and after the container has been loaded, the film can be sealed to itself through contaminants left by the product to hermetically close the container, said thermosealing layer comprising a copolymer selected from ethylene-methacrylic acid (EMAA) copolymers with methacrylic acid (MAA) content of about 4% to about 18% by weight, ethylene acrylic acid (EAA) copolymers with an acrylic acid (AA) content of from about 4% to about 22% by weight, and their blends.
Desirable properties in multilayer packaging films frequently include improved shrinkability, improved optics and improved sealability through contamination. Although multilayer packaging films in the prior art may satisfy some of the needs in the packaging art, room for improvement exists in pursuit of a packaging film with improved characteristics over materials used in the past.